Stability of Trace-Level Volatile Organic Compounds Stored in ...

Hsieh et al., Aerosol and Air Quality Research, Vol. 3, No. 1, pp.17-28, 2003

17

Stability of Trace-Level Volatile Organic Compounds Stored in Canisters and Tedlar bags Chu-Chin Hsieh*, Shih-Hwang Horng, and Pen-Neng Liao Department of Environmental and Safety Engineering, National Yunlin University of Science and Technology, Touliu, Yunlin, Taiwan, ROC Fifty-six volatile organic compounds (VOCs), known to be ozone precursors, were stored in three media (SUMMA and Silocan canisters and Tedlar bags) to evaluate their stability in these storage media. An analysis of samples of air followed the procedures described in the US EPA Method TO-15, and was performed using gas chromatograph (GC) equipped with a mass spectrometer (MS). The first-order decay model matched 87 % of the observations. These 56 VOCs were classified into four groups-alkanes, alkenes, aromatics and biogenics. Alkenes and biogenics exhibited lower recovery rates than those of alkanes and aromatics. After a seven-day (7-d) storage period, 87 % of alkenes could be recovered from canister storage and 82 % were recovered from Tedlar bag storage. Isoprene, a major component in biogenic VOCs, exhibited a recovery rate of only 75±8 % after storage for seven days in canisters and Tedlar bags. Storage conditions (humidity and temperature) affected the degradation constant of each VOC. The calculated average half-lifes of 56 VOCs for SUMMA canisters, Silcocan canisters, and Tedlar bags were 45±6, 52±6, and 37±4 days, respectively. The VOCs stored in Tedlar bags had a lower recovery than those stored in canisters. Keywords: air samples, chromatography, mass spectrometry, first-order decay, recovery

1. Introduction Many diverse media are used for collecting gaseous or whole air samples, including for example, stainless steel canisters and Tedlar bags. Different analytical methods may require different sampling media. The Compendium Method TO-14 targets 41 VOCs (U.S. EPA, 1988). These compounds have been successfully stored in canisters over periods of several days to months at ppbv (parts per billion by volume) levels. Method *Corresponding author: Tel: +886-5-534-2601 ext. 4413 Fax: +886-5-531-2069 E-mail address: [email protected]

TO-15 (U.S. EPA, 1997) is used to measure 97 VOCs that represents 189 hazardous air pollutants (HAPs) listed in the Clean Air Act Amendment of 1990. Method TO-14 has been used to measure non-polar VOCs, while TO-15 is used for both non-polar and polar VOCs. The stability of several compounds to which Method TO-15 applies, has been evaluated under different storage conditions. The loss of VOCs may be due to the physical adsorption of VOCs on canister walls, the dissolution of VOCs in water condensed in the canisters, chemical reaction, hydrolysis and biological degradation (Coutant, 1993). To support calibration and performance audits, the stability of 39 and 34 VOCs in compressed gas cylinders was determined from 1 ppbv to 10 ppmv

18


(parts per million by volume) and 5 to 700 ppmv, respectively (Jayanty et al; 1992). The results indicate that the concentrations of several VOCs varied by under 10 % over several months, but that several compounds were found to be unstable at the ppbv or even the ppmv level. The unstable compounds at the ppbv level were ethylene oxide, propylene oxide and trans 1,4-dichloro-2-butene. Six other compounds were found to be unstable in compressed gas cylinders at the ppm level (aniline, cyclohexane, p-dichlorobenzene, ethylamine, 1,2-dibromoethylene, and formaldehyde). A review of the literature on the stability of 52 HAPs stored in stainless steel canisters showed that canister stability data were widely variable for a dozen compounds (Kelly and Holdren, 1995). The stability of 194 compounds stored in SUMMA canisters at the ppbv level was examined (Brymer et al, 1996). Although the concentrations of 168 of the 194 compounds studied did not change, a few of the compounds studied including methyl mercaptan, ethyl mercaptan, butyl mercaptan, dimethyl acetal and bis ether) exhibited high variability, low recovery or poor storage stability. Seven aldehydes and four terpenes stored in SUMMA canisters were recovered at concentrations of 3-5 ppbv (Batterman et al., 1998). The concentrations of all terpenes and most aldehydes decreased markedly during the first hour. On the 16th day, the recovery of most compounds was reduced to 50% of the initial concentration. An alternative method for collecting air samples involves Tedlar bags. Tedlar bags are used according to EPA Method 18 for measuring the emission of VOCs from high-concentration process streams (U.S. EPA, 1993). The bags are made of a chlorinated substance and are resistant to corrosion by most solvents and chemicals. However, some VOCs have been reported to degrade significantly. (For example, 70 %of methanol degrades after being stored in 10L Tedlar bags for 6 hrs and ethylbenzene and o-xylene degrades to 13% and

11% concentrations, respectively after 2 days) (Lipari, 1990). In contrast, methanol and formaldehyde stored in 60L Tedlar bags tend to degrade slowly (Andino and Butler, 1991), apparently because they have a smaller surfaceto-volume ratio than other substances. The accuracy, convenience and flexibility of canister or bag sampling can yield useful results in several contexts. This study extends previous studies on stability of compounds in storage, by assessing the storage stability of 56 VOCs (referred to as ozone precursors) stored in SUMMA canisters, Silcocan canisters and Tedlar bags. The recovery rates at two temperatures and two humidity conditions, were compared to determine the performance of each method of storage.

2. Materials and Methods The research included the preparation and storage of these 56 VOCs. These 56 VOCs are classified into four categories-alkanes, alkenes, aromatics and biogenics. Table 1 lists 56 VOCs along with their CAS numbers, boiling points, method detection limits (MDL), and whether they are listed in EPA Methods TO-14 and TO-15. Only nine of these 56 compounds have been evaluated in EPA Methods. The effects of two RHs (30 and 90 percent) and two temperatures (25 and 35℃) on the stability of the stored compounds were investigated. Triplicate samples were used. VOCs were prepared at ppbv levels (5-30 ppbv) and stored in 6L sampling media, including 12 SUMMA canisters (Anderson Samplers Inc., Atlanta, GA), 12 Silcocan canisters (RESTEK Co., Bellefonte, PA) and 12 6L Tedlar bags (SKC Inc., Eight y Four, PA). According to the manufacturer’s information, the interior of each S U M M A  c a ni s t e r w a s s u bj e ct e d t o a n electropolishing treatment to reduce the surface adsorption of VOCs. Each silcocan canister had been subjected to siloxane to impart resistance and


19

Table 1. Properties of targeted VOCs

Biogenics

Aromatics

Alkanes

Alkenes

Cat.

VOC Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56

VOCs 1-Butene trans-2-Butene cis-2-Butene 3-Methyl-1-Butene 1-Pentene trans 2-Pentene cis 2-Pentene 2-Methyl-2-Butene Cyclopentene 4-Methyl-1-Pentene 2-Mrthyl-1Pentene trans-2-Hexene cis-2-Hexene Isobutane n-Butane Isopentane n-Pentane 2,2-Dimethylbutane Cyclopentane 2,3-Dimethylbutane 2-Methylpentane 3-Methylpentane n-Hexane Methylcyclopentane 2,4-Dimethylpentane Cyclohexane 2-Methlyhexane 2,3-Dimethylpentane 3-Methylhexane 2,2,4-TMP n-Heptane Methylcyclohexane 2,3,4-TMP 2-Methylheptane 3-Methylheptane n-Octane n-Nonane Benzene Toluene Styrene Ethylbenzene m,p-Xylene o-Xylene Isopropylbenzene n-Propylbenzene 1,3,5-TMB 1,2,4-TMB Isoprene alpha-Pinene beta-Pinene Methacrolein Hexanal 2-Heptanone p-Cymene 1-Decene 1-Dodecene

CAS-No.

MW

BP.

106-98-9 624-64-6 590-18-1 563-45-1 109-67-1 646-04-8 627-20-3 513-35-9 142-29-0 691-37-2 763-29-1 4050-45-7 7688-21-3 75-28-5 106-97-8 78-79-5 109-66-0 75-83-2 287-92-3 79-29-8 107-83-5 96-14-0 110-54-3 96-37-7 108-08-7 110-82-7 591-76-4 565-59-3 589-34-4 540-84-1 142-82-5 108-87-2 565-75-3 592-27-8 589-81-1 111-65-9 111-84-2 71-43-2 108-88-3 100-42-5 100-41-4 108-33-3/106-42-3 95-47-6 98-82-8 103-65-1 108-67-8 95-63-6 78-79-5 80-56-8 127-91-3 78-85-3 110-54-3 110-43-0 99-87-6 872-05-9 112-41-4

56.1 56.1 56.1 70.1 70.1 70.1 70.1 70.1 68.1 84.2 84.2 84.2 84.2 58.1 58.1 72.0 72.2 86.2 70.1 86.2 86.2 86.2 100.0 84.2 100.2 84.2 100.2 100.2 100.2 114.2 100.2 98.2 114.2 114.2 114.2 114.0 138.0 78.1 92.1 104.2 106.2 106.2 106.2 120.2 120.2 120.2 120.2 68.1 136.2 136.2 70.1 100.0 114.0 134.0 140.0 168.0

-6.25 1.0 3.7 20.0 30.0 36.0 37.0 35.0 68.1 53.0 53.0 62.0 62.0 -12.0 -0.5 34.0 36.0 50.0 49.5 57.9 60.0 63.0 69.0 72.0 81.0 81.0 90.0 90.0 100.0 99.0 98.0 101.0 112.0 118.0 119.0 126.0 151.0 80.0 110.6 145.2 136.0 138.5 144.4 152.4 159.0 165.0 169.0 34.0 155.5 164.0 68.4 128.6 151.5 176.5 172.0 213.0

TO-14A

TO-15

×

× × × × × × × ×

MW: molecule weight (g); BP: boiling point (℃); TMP: trimethylpentane; TMB: trimethylbenzene

MDL (ppb) 0.4 0.4 0.6 0.4 0.6 0.5 0.6 0.5 0.4 0.3 0.4 0.7 0.5 0.5 0.5 0.5 0.6 0.4 0.5 0.6 0.4 0.5 0.6 0.6 0.5 0.7 0.6 0.5 0.4 0.5 0.5 0.5 0.6 0.6 0.5 0.5 0.6 0.4 0.6 0.4 0.5 0.7 0.6 0.7 0.5 0.6 0.5 0.5 0.5 0.5 0.7 0.5 0.6 0.4 0.6 3.8

Calibration R2 0.995 0.998 0.998 0.995 0.997 0.997 0.999 1.000 0.995 0.999 0.996 0.998 0.998 0.995 0.998 0.998 0.999 0.999 0.995 0.999 0.999 0.996 0.999 0.995 0.999 0.997 1.000 0.995 1.000 0.996 0.999 0.995 1.000 0.995 0.999 0.998 0.998 0.997 0.999 1.000 0.995 0.998 0.998 1.000 0.998 0.999 1.000 0.999 0.997 0.999 0.996 0.998 0.996 0.998 0.999 0.995

20


improve sample-wetting characteristics. The TO-14 canister cleaning procedure consisted of repeated cycles of pressurization with humidified zero air, followed by evacuation to 0.1 torr for 1h, with heating to 120℃. Each canister was first filled with humidified zero air and analyzed by the procedure described below to determine its cleanliness. Samples prepared according to the procedures of Batterman et al. (1998). Clean evaluated canisters were humidified by injecting 69 µl and 207 µl of HPLC-grade water and then filled with dry zero air to near atmospheric pressure, to make up 30 and 90% RH at 25 ℃, respectively. Volumes of 108 µl and 323 µl of HPLC-grade water were injected into clean evaluated canisters to yield 30 and 90 % RH at 35 ℃, respectively. The samples thus prepared were stored in an oven to maintain the target temperature. Organic compounds were prepared using a gas standard (Environ-Mat Ozone Precursor Mixture, Matheson Gas Products, Georgia, USA) and biogenic compounds were prepared using a liquid standard (Supelco, PA, USA). The gas sample was diluted with dry zero air in Tedlar bags and then transferred into stainless steel canisters. Liquid samples were diluted in methanol. Then, the mixture was injected into Tedlar bags and canisters, with proper dilution. All samples thus prepared were heated using an infrared lamp to keep VOC in the vapor phase. Most samples were prepared at concentrations (5 to 30 ppbv) similar to ambient levels. All Tedlar bags were kept in black bags to prevent photochemical reactions. The procedures described in Method TO-15 were followed to analyze the samples. Samples of air were concentrated cryogenically using a preconcentrator (7100, Entech Instrument, Inc., CA) and transferred by a heated transfer line and a cryofocus unit to a gas chromatograph (GC) (5890 SeriesⅡplus, Hewlett Packard, Palo Alto, CA). The flow of a 400ml sample of air was metered by a mass-flow-controlled meter (Sierra Instrument

Inc., Monterey, CA). The GC used a HP-1 60m capillary column (0.25-mm diameter x 1.8-µm film thickness, Hewlett Packard) and nitrogen as the carrier gas. VOCs were detected by massspectrometry (MS) (Model 5972, Hewlett Packard). The MS scanned a wide range of mass to charge ratios (40-200). A PC workstation (G1034C Chemstation, Hewlett Packard) was used to acquire and manipulate data. The starting temperature in the GC oven was 10 ℃ and was maintained for 3 min. The temperature was then increased at 8℃ per min to 130℃ , at which it was maintained for 5 min. Subsequently, the temperature was increased at 5 ℃ per min to 180 ℃ , at which it was maintained for 5 minutes. Quality control and quality assurance requirements involved establishing GC retention times, calibration curves and MDLs, as well as studying the reproducibility of the results for all compounds analyzed. Furthermore, an internal standard (toluene-d8) was used for each analysis to verify the stability of the MS. Additionally, cyclohexanal-d12 was analyzed daily as a reference standard for determining the performance of the instruments used. A response deviation of more than 15% resulted in full instrument recalibration. Good results were achieved. Table 1 shows MDL and linearity coefficient (R2) for each VOC. Triplicate samples were prepared and analyzed, so the reported concentrations are the average measured concentrations of the compounds.

3. Results and Discussion This study has investigated at two humidity levels (30 and 90% ) and two temperatures (25 and 35 ℃), representative of environmental conditions in Taiwan island affect the stability and recovery of stored compounds. Thus, four storage conditions, as determined by temperature and RH, (25 ℃, 30 percent RH), (35 ℃, 30 percent RH), (25 ℃, 90 percent RH) and (35 ℃, 90 percent RH), were


21

Table 2. Correlation coefficients (R2>0.7) for 1st order decay under four storage conditions. Summa C1

Biogenics

Aromatics

Alkanes

Alkenes

VOC Cat. NB

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56

VOCs \ (T, RH) 1-Butene trans-2-Butene cis-2-Butene 3-M-1-Butene 1-Pentene Trans 2-Pentene cis 2-Pentene 2-M-2-Butene Cyclopentene 4-M-1-Petene 2-M-1 Petene Trans-2-Hexene cis-2-Hexene Isobutane n-butane Isopentane n-Pentane 2,2-DM-butane Cyclopentane 2-M-pentane 3-M-pentane 2,3-DM-butane n-Hexane M-cyclo-P 2,4-DMP Cyclohexane 2-M-hexane 3-M-hexane 2,3-DMP 2,2,4-TMP n-Heptane M-cyclohexane 2,3,4- TMP 2-M-heptane 3-M-heptane n-Octane n-Nonane Benzene Toluene Styrene Ethylbenzene m,p-Xylene o-Xylene Isopropyl-B n-Propyl-B 1,3,5-TMB 1,2,4-TMB Isoprene Alpha-Pinene beta-piene Methacrolein Hexanal 2-Heptanone p-Cymene 1-Decene 1-Dodecene

Silcocan C2

Tedlar Bags (TB)

(25,30) (35,30) (25,90) (35,90) (25,30) (35,30) (25,90) (35,90) (25,30) (35,30) (25,90) (35,90) 0.73 0.74 0.75 0.87 0.92 0.92 0.98

0.84 0.88 0.93

0.87 0.87 0.93 0.83 0.82 0.92 0.94 0.88

0.80 0.80 0.86 0.78 0.75 0.72 0.92 0.71 0.82

0.95 0.72 0.78

0.95 0.72 0.78 0.89 0.77 0.85 0.71 0.90 0.81 0.73 0.88 0.98 0.95 0.88 0.93 0.93 0.86

0.80 0.72

0.90 0.61 0.79 0.85 0.75 0.72 0.71 0.72 0.70 0.72 0.93 0.74

0.82 0.77 0.90

0.93 0.74 0.72

0.73

0.71 0.76 0.80 0.80

0.90 0.80 0.77 0.97 0.97 0.74 0.70 0.93 0.86 0.77 0.83 0.85 0.96 0.93 0.80 0.92 0.91 0.94 0.82

0.74 0.77 0.85 0.73 0.92 0.83 0.71 0.74 0.87 0.95 0.83 0.92 0.88 0.71 0.72

0.93 0.84 0.89

0.94 0.87 0.90 0.95 0.95 0.91 0.94 0.95 0.94 0.81 0.81 0.74 0.76

0.82 0.86 0.90 0.93 0.83 0.83

0.92 0.89 0.82 0.75 0.73 0.88 0.92 0.71 0.71 0.91 0.90 0.96 0.93 0.81 0.81

0.89 0.83 0.91 0.72 0.73

0.87 0.81 0.93 0.90 0.95 0.88

0.93 0.75 0.88 0.72

0.75 0.92

0.93 0.77 0.89 0.81 0.96 0.94 0.72 0.89

0.92 0.86 0.71 0.70 0.95

0.88 0.90 0.91 0.88 0.84 0.96 0.74 0.83 0.74

0.77 0.83 0.82

0.72 0.72 0.90 0.83 0.90 0.86 0.90 0.89 0.82 0.92 0.97

0.86 0.82 0.91 0.73 0.85 0.84 0.94 0.90 0.94 0.72 0.82 0.71 0.75 0.90 0.73 0.92 0.83 0.73 0.79

0.89 0.81 0.77 0.85

0.90

0.94 0.89

0.93 0.90 0.89 0.72

0.93 0.87 0.80 0.80

0.81 0.72

0.87 0.94 0.87

0.86 0.95 0.89 0.86 0.71 0.82 0.85 0.92 0.89 0.85 0.92 0.82 0.88

0.98 0.89 0.98 0.92 0.85 0.94 0.94 0.93 0.93 0.89 0.88 0.92 0.89

0.75 0.90 0.83 0.85 0.96 0.88 0.92 0.94 0.79 0.86 0.92 0.89 0.85

0.72 0.83

0.81 0.91 0.83 0.91 0.87 0.75 0.80 0.85 0.92 0.90 0.81 0.89 0.92 0.85

0.85 0.98 0.93 0.71 0.83 0.88 0.86 0.95 0.92 0.89 0.70 0.83 0.88 0.82 0.93 0.88 0.87 0.81 0.81

0.88 0.81 0.81 0.74 0.89 0.93 0.86 0.83 0.78 0.78 0.87 0.87 0.92 0.88 0.91 0.85 0.86 0.90

0.93

0.92

0.77

0.87

0.81

0.97

0.81

0.78

0.93

0.88

0.92

0.80 0.86 0.92 0.75

0.82 0.71 0.90 0.87

0.86 0.88 0.93 0.91

0.78 0.95 0.80 0.87

0.90 0.73 0.71 0.84

0.86 0.77 0.88 0.80

0.69 0.73 0.75 0.69

0.87 0.84 0.89

0.79 0.89 0.87 0.89

0.86 0.92 0.81 0.86

0.89 0.71 0.95 0.89

0.91 0.98 0.93 0.95 0.88 0.90 0.76

0.87 0.86 0.86 0.83 0.87 0.89 0.88 0.87 0.82

0.78 0.85 0.75

0.81 0.72 0.91 0.87 0.80 0.78

0.97 0.86 0.91 0.88 0.87 0.79

0.85 0.83

0.83 0.71 0.85 0.90 0.95 0.92 0.74 0.85 0.94 0.85

0.97 0.83 0.73

0.74 0.78 0.76

0.91 0.76 0.86 0.83 0.91 0.87 0.97 0.96 0.94 0.85

0.91 0.90 0.91 0.94 0.96 0.95 0.94 0.93 0.94 0.94

0.96 0.81 0.77 0.87 0.91 0.74 0.88 0.90 0.95 0.91

0.92 0.93 0.93 0.89 0.92 0.96 0.96 0.89 0.92 0.93

0.88 0.90 0.82 0.82

0.75 0.79 0.85 0.77

0.87 0.91 0.74

0.90 0.82 0.92 0.74 0.85 0.72 0.85 0.79 0.73

0.71 0.77

0.74 0.83 0.70 0.96 0.81 0.94 0.86 0.92 0.77

0.74 0.86 0.90 0.87 0.96 0.95 0.94 0.92 0.96

0.70 0.89 0.97 0.86 0.82 0.88 0.86 0.89 0.90

0.96 0.74 0.82 0.96 0.82 0.91 0.96 0.89 0.94 0.85 0.99 0.85 0.94 0.79 0.81 0.82 0.94 0.88

0.90 0.86 0.85 0.93 0.88

0.79 0.74

0.74 0.79 0.76 0.87 0.90

0.76

0.82 0.88 0.81

0.83 0.74

0.88 0.83 0.72 0.67 0.75 0.88 0.86 0.83

0.86

0.77 0.89 0.76

0.70 0.82 0.94 0.73 0.78

RH: relative humidity (%), B: benzene, M: methyl, DM: dimenthyl, TM: trimethyl, B: butane, P: pentane, NB:VOC number

0.88 0.96 0.96 0.84 0.91 0.94 0.96


22

4.0

n-Propylbenzene Isoprene

3.5

Alkenes

2

R = 0.97

10

2

R = 0.93

2

R = 0.92 2

R = 0.90 0 0

5

10

15

20

25

30

Elapsed time (d)

Decay rate constant ( k, 100 d-1)

Methylcyclohexane cis-2-Butene

3.0

Alkanes Aromatics

2.5

Biogenics

2.0

Average 1.5 1.0 0.5

Figure 1. Concentration against time on a semi-log plot.

0.0 C1 (2 5, C1 30) (3 5, C1 30) (2 5, C1 90) (3 5, C2 90) (2 5, C2 30) (3 5, C2 30) (2 5, C2 90) (3 5 TB ,90) (2 5, TB 30) (3 5 TB ,30) (2 5 TB ,90) (3 5, 90 ) A ve ra ge

Concentration (ppbv)

20

Environmental conditions

considered. The first-order decay model of VOC recovery from sampling media may be applicable to most compounds. The first-order decay model is expressed as Ct / Co= exp (-k t)

(1)

where Co represents the initial concentration; Ct is VOC concentration at a storage time of t (d), and k is the degradation constant (d-1). The ratio of Ct to Co in Eq.1 is defined as the recovery rate, which is typically used to evaluate the stability of compounds stored in the containers over a certain period. Figure 1 shows typical VOC recovery variations with time for one compound in each category of VOCs. Recoveries refer to the initial analysis of each canister, and are averaged across the three sets of samples. Table 2 summarizes the correlation coefficients for fitting the first-order decay model under the four storage conditions. Clearly, several compounds have relatively high R2 values. Observations of R2 below 0.7 were excluded from the table as the low R2 indicated a poor fit of the first-order model. Overall, the first-order model explained 87% of the observations. Tedlar bags yielded a higher R2 than the canisters, perhaps because the first-order model fits fast processes better, such as are supported by Tedlar bags.

Figure 2. Decay rate constants for four categories of VOCs under various storage conditions.

3.1 First-order Decay Kinetic Constant The first-order model of VOC decay in storage media may be valid for several compounds, at least over the period during which recoveries slowly decline. Table 3 shows the first-order degradation constants for those compounds with R2 > 0.7. Low R2 values indicate that the estimated decay kinetic constant is not significantly related to errors in the analytical determinations. The highest and lowest k values are obtained at 35℃ with 30 % RH and 25℃ at 90 % RH, respectively. The VOCs stored in Tedlar bags tend to exhibited higher k values. Samples stored in canisters at high humidity and low temperature are generally more stable than those stored in canisters at low humidity and high temperature. For illustration, Figure 2 shows the average k values of categorized VOCs. Among the four categories of compounds, biogenics exhibit the highest k values, and alkanes the lowest; aromatic VOCs have k values close to the average k value of the 56 compounds. Since the k values of compounds stored in SUMMA and Silcocan canisters do not statistically significantly different, Table 4 presents the mean k values for the two kinds of canisters was tabulated, and compared


23

Table 3. Degradation constant (k, 100 d-l) of 56 VOCs in three media for R2＞0.7 Summa C1

Biogenics

Aromatics

Alkanes

Alkenes

Cat.

Silcocan C2

Tedlar Bags (TB)

VOC VOCs \ (T, RH) (25,30) (35,30) (25,90) (35,90) (25,30) (35,30) (25,90) (35,90) (25,30) (35,30) (25,90) (35,90) NB 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56

1-Butene trans-2-Butene cis-2-Butene 3-M-1-Butene 1-Pentene Trans 2-Pentene cis 2-Pentene 2-M-2-Butene Cyclopentene 4-M-1-Petene 2-M-1 Petene Trans-2-Hexene cis-2-Hexene Isobutane n-butane Isopentane n-Pentane 2,2-DM-butane Cyclopentane 2-M-pentane 3-M-pentane 2,3-DM-butane n-Hexane M-cyclo-P 2,4-DMP Cyclohexane 2-M-hexane 3-M-hexane 2,3-DMP 2,2,4-TMP n-Heptane M-cyclohexane 2,3,4- TMP 2-M-heptane 3-M-heptane n-Octane n-Nonane Benzene Toluene Styrene Ethylbenzene m,p-Xylene o-Xylene Isopropyl-B n-Propyl-B 1,3,5-TMB 1,2,4-TMB Isoprene Alpha-Pinene beta-piene Methacrolein Hexanal 2-Heptanone p-Cymene 1-Decene 1-Dodecene

1.7 2.0 1.9 2.4 2.1 2.6 2.9

2.4 2.2 2.1 1.7 0.8 0.9 1.3 1.4 1.5 1.2 1.5 1.5 1.0 1.1 1.4 1.2 1.1 1.2 1.1 1.0 1.2 1.1 1.4 1.2 1.3 1.2 1.3 1.7 1.6 1.9 1.8 1.4 1.9 2.3 2.3 3.2 2.2 2.0 3.3 2.9 3.6 2.1 2.8

2.8 2.8 3.2 2.3 3.6 2.8 2.5 2.3

2.3 2.0 1.9 2.0 1.5 2.2 2.3 2.3 2.2

2.8 2.2 2.8 0.9

1.0 1.2 1.2 1.2 1.5 1.5 1.3 1.1 1.3 1.2 1.2 1.2 1.3 1.2 1.4 1.3 1.5

3.3 1.7 2.0 2.1 1.5 3.9 3.5 5.3 2.5 2.4 3.5

1.7 1.4 0.8 0.9

0.9 1.0 0.8

0.9

1.1 1.2 1.0 1.0

1.1 1.7 1.0 1.1 1.9 1.3 1.5 1.5 0.9 1.1 1.7 2.2 3.0 2.1 1.9 3.2

1.8 2.8 1.7 2.7 2.6 2.3 2.6 2.0 7.7 2.6 1.7 1.9 1.8 1.0 1.0 1.3 1.9 1.4 1.2

1.2 1.2 1.2 1.1 1.4 1.2 1.3 1.3 1.2 1.2 1.2 1.5 1.3 1.7 1.4 1.4 2.1 1.6 1.8 1.8 1.3

4.6 2.4 2.2 3.5

4.1 2.4 2.2 3.3

2.1

2.4 2.2 3.0

1.7 1.5 2.4 1.7 2.9 2.6 2.8 2.3 1.9 2.0 1.7 1.4 0.7 0.8 0.8 1.1 1.1 1.6 0.8 0.9 0.8 0.9 1.1 1.1 1.0 1.2 0.9 1.0 1.1 1.1 1.2 1.2 1.2 1.0 1.2 2.3 1.2 1.6 1.4 1.2 1.4 2.2

2.0 2.8 1.9 2.2 2.8 2.6 3.0 3.0 3.3 2.7 2.3 1.9 2.6 1.2 1.4

1.2 1.2 1.1 1.7

1.4

1.1 1.0 1.1 1.1 1.2 1.3 1.2 0.9 0.9 1.3 2.2 1.7 1.7

7.7 1.9 1.9 3.5 3.2 1.9 1.6 2.2

2.0

3.3 2.4 1.6

1.7 1.3 1.4 1.2 1.7

1.9 2.0 2.0 1.7 1.9 1.5 1.4 0.7 0.7 0.6 0.9 0.9 0.9 0.8 0.9 0.9 0.8 1.0 1.0 0.9 1.0 0.9 0.9 0.9 1.0 1.0 1.1 1.0 0.9 1.0 1.5 1.1 1.4 1.5 1.3 1.7 2.0 2.2 1.8 1.7 3.0 2.7 2.9 1.5 1.4 1.8

2.3 2.4 2.0 2.6

3.6 2.9 3.2 2.6

1.3 2.0

3.3 2.9 2.1 2.3 4.3 2.6 2.8 1.9 1.9 1.4 2.3 1.4 0.9 2.1 2.5 1.1 1.5

1.7 1.3 2.0 1.9 2.2 1.5 0.9 1.3 1.0 0.8 1.0 1.4 1.0 1.5 0.9 1.0 1.1 1.0 1.2 1.1 1.1 1.1 0.9 1.0 0.9 1.1 1.3 1.1 1.1 1.1 1.1 2.0 1.4 1.7 1.6 1.7 2.2 2.5 3.6 2.2 3.2 2.8 3.9 1.7 1.3

1.1 1.1

1.3 1.5 1.4 1.3 1.7 1.5 1.4 1.1 1.1 2.5 1.8 1.9 1.7 1.7 2.4 2.4 2.8 3.0 2.5 2.8 3.6 3.5 3.5 2.2 2.4 2.5

4.6 3.0 3.2 3.3 1.2 2.7 2.6 2.0 3.3 3.3 2.8 5.3 3.6 3.2 1.5

1.6 2.2 1.9 2.5 1.5 1.2 1.7 1.5 2.0 1.7 1.5 1.9 2.2 1.6 1.8 1.7 1.4 1.9 1.8 1.3 1.7 3.3 2.0 2.5 2.4 1.9 2.5 2.6 3.5 5.3 2.6 3.0 3.6 3.6 3.9 2.2 2.7 3.0

2.6 3.0 3.0 2.6 2.3 2.6 2.6 2.3 2.0 2.3 2.3 2.3 2.1 1.7 2.8 1.0 1.0 1.0 1.2 1.3 1.9 1.4 1.3 1.7 1.0 1.1 1.1 1.2 1.2 1.2 1.4 1.1 1.5 1.3 1.2 1.4 1.7 1.2 1.3 2.2 1.7 1.4 1.7 1.3 2.0 2.6 2.9 3.3 2.3 2.3 3.6 3.2 3.3 1.7 1.8 2.6

2.5 3.5 4.6 3.0 3.5 3.5 2.7 3.3 1.9 5.0 3.0 3.5 2.7 1.8 1.1 2.0 1.2 0.9 1.6 1.4 2.0 1.6 1.3 1.4 1.3 1.6 1.4 1.4 1.6 1.9 1.3 1.7 1.7 1.2 1.7 1.7 1.3 1.9 2.4 2.0 2.1 2.2 1.9 2.4 2.6 2.9 2.5 3.0 3.5 3.9 1.7 2.9 2.8

RH: relative humidity (%), B: benzene, M: methyl, DM: dimenthyl, TM: trimethyl, B: butane, P: pentane, NB:VOC number.


24

100 Alkenes

Alkanes

Aromatics

Biogenics

Percent recovery

95 90 85 80 75 70 0

10

20

30

40

50

60

C1(25,30) C1(35,30) C1(25,90) C1(35,90) C2(25,30) C2(35,30) C2(25,90) C2(35,90) TB(25,30) TB(35,30) TB(25,90) TB(35,90)

VOC number

C1= Summa canisters, C2: Silcocan canisters, TB: tedlar bags

Figure 3. Recovery rates of 56 VOCs under variousstorage conditions after seven days them with those obtained for Tedlar bags under the four storage conditions. The effect of humidity on the stability of samples has been reported elsewhere (MacClenny et al., 1991; Batterman et al., 1998): water vapor promotes sample stability because it fills active adsorption sites on the canister’s interior surface, reducing the adsorption of VOCs.

Tedlar bag). Of the four categories, the highest recovery was obtained for alkanes (middle in Fig. 3) and lowest was of biogenics. Isoprene, as one of the most importantly reactive hydrocarbons in atmospheric chemistry, with a boiling point of 34 ℃, had the lowest recovery (75 ± 8 percent) rate of any of the nine biogenics evaluated in this study.

3.3 Half-life Estimates 3.2 7-day Recovery Estimates A storage period of seven days was used for evaluating stability since this is the period typically required to complete collection, transportation and analysis of such samples. Recoveries are obtained from the initial analysis of each sample, and are averaged across the three samples. The decay kinetic constants in Table 3 were used to estimate the recovery of the VOCs with R2 > 0.7. The results shown in Fig. 3 show that the recovery of each VOC varies widely among the three media (SUMMA canister, Silcocan canisters and

The VOC decay kinetic constants can be further used to estimate the compound’s half-lives, which are defined as the time for 50 % of the initial concentration to disappear. Figure 4 shows the calculated half-life (τ = 0.693/k) of VOCs stored in three types of containers (SUMMA canister, Silcocan canister and Tedlar bag). The half-life data summarize VOC stability, and support straightforward calculation of the maximum storage times may match daily drift expressed as, “percent change per day (U.S. EPA, 1988)”. The calculated mean half-lives of 56 VOCs in SUMMA


25

12 Alken

Alkan

Aroma

Bioge

half-life (d)

10 C1(25,30) C1(35,30) C1(25,90) C1(35,90) C2(25,30) C2(35,30) C2(25,90) C2(35,90) TB(25,30) TB(35,30) TB(25,90) TB(35,90)

8

6

4

2

0 0

10

20

30

40

50

60

VOC number C1=Summa canisters, C2=Silcocan canisters, TB: tedlar bags Figure 4. Half-life of 56 VOCs under various storage conditions. canisters, Silcocan canisters, and Tedlar bags were 45±6, 52±6 and 37±4 days, respectively. The half-lives could be used to calculate the maximum storage periods. However, such estimates may be too low because losses at the beginning of the storage and the effects of old or damaged canisters are neglected; rather, the estimates apply under laboratory conditions. Thus, a “safety factor” should be included, by, say, halving the estimated maximum storage time (Batterman et al., 1998).

3.4 Inter-canister Reproducibility Reproducibility was calculated as the average coefficient of variation (COV) of concentrations measured among the three types of canisters (or bags) for each set of samples. Table 5 shows the mean COV for each of the four groups under different storage conditions. Reproducibility was in all cases between 9 and 30%. The reproducibility

during the study period COVs observed under storage conditions of high temperature and low humidity. Among the four categories, biogenic had the highest COVs. The preconcentrator/GC/MS system used in this study appeared to meet the performance requirements of TO-14 and TO-15, using a standard calibration of R2 > 0.995, meeting precision requirements within 25% (U.S. EPA, 1988 & 1997). The COVs in Table 5 incorporate both analytical uncertainties and differences among canisters. The higher uncertainties may have resulted from variations in active sites on surfaces of the sampling media, differences among the amounts of gas transferred from the highly concentrated standard, unequal pressurization of canisters and other factors (Wagoner et al., 1993).

3.5 Loss Mechanisms

26


Table 4. Degradation constant (k, 100 d-1) of four categories VOCs under different environmental conditions. Canisters Tedlar Bags RH VOCs 30 90 30 90 T 2.2 1.8 2.8 2.5 25 (0.5) (0.3) (0.7) (0.3) Alkenes 2.6 2.3 3.1 3.3 35 (0.5) (1.0) (1.0) (0.8) 1.1 1.0 1.4 1.4 25 (0.2) (0.2) (0.4) (0.4) Alkanes 1.3 1.2 1.8 1.5 35 (0.4) (0.2) (0.5) (0.3) 1.7 1.4 1.9 1.8 25 (0.5) (0.4) (0.6) (0.6) Aromatics 2.0 1.8 2.4 2.2 35 (0.5) (0.5) (0.7) (0.4) 2.6 2.4 2.9 2.7 25 (0.7) (0.6) (0.5) (0.7) Biogenics 3.3 2.8 3.3 3.2 35 (1.8) (0.9) (0.9) (0.9) T: Temperature (℃), RH: Relative Humidity (%) ( ): Standard deviation

VOC losses in canisters and Tedlar bags may proceed by several possible mechanisms. Physical adsorption is the first loss mechanism. Rapid declines in recoveries over the first few hours or days of storage are normally associated with physical adsorption, although such losses may be related to chemical reactions (U.S. EPA, 1997). Adsorption was considered as the chemical interaction of vapors on “active sites” at the canister walls (Freeman et al., 1994). The results imply that Silcocan canisters had fewer active sites and thus adsorbed less. Tedlar bags gave more active sites than canisters, and thus absorb more. A second mechanism that affects recovery is the dissolution of soluble compounds in condensed water present in the canister. As the pressure in a canister drops because of withdrawal of a sample, the quantity of condensed water falls and concentrations of dissolved compounds may thus

Table 5. Average reproducibility of four groups under for storage conditions expressed as COV in percent. SUMMA Silcocan Tedlar canisters canisters bags RH VOCs 30 90 30 90 30 90 T 25 18 13 17 13 21 16 Alkenes 35 21 15 20 13 25 21 25 13 9 12 10 15 11 Alkanes 35 15 12 14 11 16 14 25 12 9 11 9 14 12 Aromatics 35 14 11 13 10 15 13 25 24 18 23 18 28 23 Biogenics 35 28 21 28 22 30 25 Average 18 14 17 13 21 17 T: Temperature (℃), RH: Relative Humidity

increase (Coutant, 1993). This effect depends on the compound’s solubility and the presence of condensed water in the canister. A third mechanism involves a gas-phase reaction and the subsequent transformation and loss of objective compounds. Such a reaction may be promoted by the presence of ozone or other reactive species. Information available in the literature and in this study is insufficient to identify the details of the loss mechanisms. A general mechanism that affects all tested compounds is expected since compounds stored at a lower temperature are more stable and their are more reproducible, probably because the reaction rates are lower. Moreover, adding water or increasing humidity may also reduce adsorption losses of several VOCs. The low recoveries in dry canisters verify the occurrence of physical adsorption. Those storage conditions applied in this study were limited to typical environmental conditions in the Taiwan area. Since the potential for adsorbing VOCs and water in a canister changes with temperature, recovery must be further investigated over a range of temperatures (such as 0-40 oC) and humidities, to separate out sorption losses.


4. Conclusions Storage conditions importantly affect the decay kinetic constant of VOC mixtures. The results indicate that a stored compound exhibits a higher recovery and half-life at lower temperature and a higher RH. Most alkenes and biogenics exhibit lower recoveries and half-lives than alkanes and aromatics. Not all sampling media should be used as absolute references for mixtures of some biogenics (especially isoprene). Organic compounds stored in Silcocan canisters are slightly more stable than those stored in SUMMA canisters, but relatively more stable than those in Tedlar bags. The short-term degradation of samples with low concentrations of VOCs cannot be ignored since sample degradation is noted early on during storage. Thus, the holding period between VOC collection and analysis should be minimized to improve the recovery of VOC mixtures.

Acknowledges We gratefully acknowledge the financial support of the Taiwan EPA (EPA-88-SA21-03-0012 and EPA-89-FA11-03-100). This paper has not been subject to Taiwan EPA policy review and should not be construed to represent the policy of agency.

References Andino J. M. and Butler J. W. (1991), A Study of the Stability of Methanol-Fueled Vehicle Emissions in Tedlar Bags. Environ. Sci. Technol. 25: 1644-1646. Batterman S. A., Zhang G. Z. and Melissa B. (1998), Analysis and Stability of Aldehydes and Terpenes in Electropolished Canister. Atmos. Environ. 32: 1647-1655. Brymer D. A., Ogle L. D., Jones C. J. and Lewis D. L. (1996), Viability of Using SUMMA Polished Canister for the Collection and Storage of Parts

27

Per Billion by Volume Level Volatile Organics. Environ. Sci. Technol. 30: 188-195. Clean Air Act Amendments of 1990 (1990), Conference Report to Accompany S. 1630, Report No. 101-952, 139-162, U.S. Government Printing Office, Washington, DC. Coutant R. W. (1993), Theoretical Evaluation of Stability of Volatile Organic Chemicals and Polar Volatile Organic Chemicals in Canisters, U. S. Environmental Protection Agency, EPA Contact No. 68-DO-0007, Work Assignment, 45, Subtask, Battelle, Columbus, OH. Freeman R. R., Crume C. C. and Winnergar E. D. (1994), Beyond Canister Cleaning, what about the surface chemistry? Proceedings U.S. EPA/A&WMA International Symposium on the Measurement of Toxic and Related Air Pollutants, Durham, North Carolina, 656-661. Jayanty R. K. M., Albritton J. R. and Straley Y. H. (1992), Stability of ppb Organic Cylinder Gases Used for Calibration and Audits. J. Air Waste Manage. Assoc. 42: 1198-1208. Kelly T. J. and Holdren M. W. (1995), Applicability of Canisters for Sample Storage in the Determination of Hazardous Air Pollutants. Atmos. Environ. 29: 2595-2608. Lipari F. (1990), Determination of Individual Hydrocarbons in Automobile Exhaust from Gasoline-, Methanol- and Variable-Fueled Vehicles. J. Chromatogr. 503: 51-68. McClenny W. A., Pleil J. D., Evans G. F., Oliver K. D., Holdren M. W. and Winberry W. T. (1991), Canister-based Method for Monitoring Toxic VOCs in Ambient Air. J. Air Waste Manage. Assoc. 41: 1308-1318. U.S. Environmental Protection Agency (1988), Compendium Method TO-14: Determination of Volatile Organic Compounds (VOCs) in Ambient Air Using SUMMA Passivated Canister Samping and Gas Chromatography Analysis. Environmental Monitoring Systems Laboratory, U.S. Environmental Protection

28


Agency, Research Triangle Park, North Carolina. U.S. Environmental Protection Agency (1993), Method 18, Code of Federal Regulations, Part 60; Title 40; Appendix A; U.S. GPO: Washington, DC. U.S. Environmental Protection Agency (1997), Compendium Method TO-15: Determination of Volatile Organic Compounds (VOCs) in Air Collected in Specially Prepared Canister and Analyzed by Gas Chromatography/Mass Spectrometry (GC/MS)”. U.S. Environmental Protection Agency, Cincinnati, OH, EPA 625/R-96/010b.

Wagoner D. E., Rice J. and Bursey J. T. (1993), Standard Technical Approach for the Analysis of Ambient Air Standards. In Sampling and Analysis of Airborne Pollutants, eds E.D. Winegar and L. H. Keith. Lewis, Boca Raton, Florida.

Received for review, November 11, 2002 Accepted, June 8, 2003 AAQR-2003-03